Gallium is an element that is used in semiconductor and electronics industries. Gallium is generally recovered as a by-product from Bayer-process liquors containing sodium aluminate (see for example, U.S. Pat. Nos. 2,793,179 and 2,582,377). Although electrodeposition is a common method to recover bulk Ga (see for example, U.S. Pat. No. 3,904,497) out of basic or acidic solutions, or to purify bulk Ga, there have not been many applications for this material where thin films were deposited with controlled uniformity, morphology and thickness. Therefore, only a few electroplating bath chemistries and processes were developed and reported for the deposition of thin layers of Ga on substrates for electronic applications. For example, Ga-chloride solutions with pH values varying between 0 and 5 were evaluated by S. Sundararajan and T. Bhat (J. Less Common Metals, vol. 11, p. 360, 1966) for electroplating of Ga films. Other researchers investigated Ga deposition out of high pH solutions comprising water and/or glycerol. Bockris and Enyo, for example, used an alkaline electrolyte containing Ga-chloride and NaOH (J. Electrochemical Society, vol. 109, p. 48, 1962), whereas, P. Andreoli et al.(Journal of Electroanalytical Chemistry, vol. 385, page.265, 1995) studied an electrolyte comprising KOH and Ga-chloride.
The above mentioned prior-art methods and plating baths reportedly all achieved Ga film deposition. There are, however, some common problems associated with the prior-art electrochemical deposition processes. These problems include, low cathodic deposition efficiency due to excessive hydrogen generation, poor repeatability of the process, partly due to the poor cathodic efficiency, and the poor quality of the deposited films such as their high surface roughness and poor morphology. These issues may not be important for bulk Ga electroplating or for Ga films deposited for the purpose of investigating scientific topics such as deposition mechanisms. Poor film morphology or inadequate thickness control may also not be important for the electrically inactive applications of Ga layers, such as their use as lubricating coatings etc. However, properties of the Ga films become important for certain new electronic applications where Ga film plays a role in forming an active portion of an electronic device, such as a solar cell.
Prior-art Ga electroplating techniques utilizing simple electrolytes operating under acidic or basic pH values are not suitable for the above mentioned electronics applications for a variety of reasons, including that they result in poor plating efficiencies and films with rough morphology (typically surface roughness larger than about 20% of the film thickness). Gallium is a difficult metal to deposit without excessive hydrogen generation on the cathode because Ga plating potential is high. Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used on forming the hydrogen gas, rather than the Ga film on the substrate or cathode. Hydrogen generation and evolution also causes poor morphology and micro defects on the depositing films due to the tiny hydrogen bubbles sticking to the surface of the depositing film, masking the micro-area under them, and therefore impeding deposit on that micro-area. This causes micro-regions with less than optimum amount of Ga in the film stack. Poor plating efficiencies inherently reduce the repeatability of an electrodeposition process because hydrogen generation phenomenon itself is a strong function of many factors including impurities in the electrolyte, deposition current densities, small changes on the morphology or chemistry of the substrate surface, temperature, mass transfer etc. As at least one of these factors may change from run to run, hydrogen generation rate may also change, changing the deposition efficiency.
Electrodeposition of Ga out of low pH aqueous electrolytes or solutions may suffer from low cathodic efficiencies arising from the presence of a large concentration of H+ species in such electrolytes. Therefore, hydrogen gas generation may be expected to lessen at higher pH values. However, as the pH is increased in the solution, Ga forms oxides and hydroxides which may precipitate as reported in the literature. Only at extremely alkaline pH values these oxides/hydroxides dissolve as soluble Ga species. Therefore, it becomes possible to electrodeposit Ga in a bath of pH>14 containing Ga salts as was done in prior-art techniques using high concentrations of KOH and NaOH in the bath formulation. High concentrations of alkaline species, however, cause corrosion problems for the equipment as well as the cathode material itself. There is also a limit of the Ga amount that can be dissolved in the form of acidic Ga salts (GaCl3, Ga(NO3)3 etc) in such solutions before Ga starts to precipitate. Therefore, the pH needs to be adjusted again by further addition of alkaline species such as NaOH and KOH. As pointed out above, solutions comprising a large molar amount of caustics are difficult to handle and they also have high viscosity. High viscosity makes the hydrogen bubbles formed on the cathode stick more to the cathode making it very difficult to remove them by stirring or other means of mass transfer. As explained above, such gas bubbles on the cathode surface increase defectivity of the deposited Ga layer.
As can be seen from the foregoing discussion there is a need to develop new electroplating chemistries and methods that can provide high quality electrodeposited Ga layers which may be used in electronic applications such as in processing thin film solar cells.